Download An epidemic occurs when new cases of a disease, in a

An epidemic occurs when new cases of a disease, in a given human
population, and during a given period, substantially exceed
expectations.
LEARNING OBJECTIVE [ edit ]
Give examples of current epidemics
KEY POINTS [ edit ]
Epidemiologists often consider the term outbreak to be synonymous to epidemic, but the general
public typically perceives outbreaks to be more local and less serious than epidemics.
Epidemics of infectious disease are generally caused by a change in the ecology of
the host population (e.g. increased stress or increase in the density of a vector species),
a geneticchange in the parasite population or the introduction of a new parasite to a host
population.
In the 20th century three influenza pandemics occurred, each caused by the appearance of a new
strain of the virus in humans, and killed tens of millions of people.
TERMS [ edit ]
epidemic
A widespread disease that affects many individuals in a population.
infectious
Infectious diseases, also known as transmissible diseases or communicable diseases, comprise
clinically evident illness (i.e., characteristic medical signs and/or symptoms of disease) resulting
from the infection, presence, and growth of pathogenic biological agents in an individual host
organism.
population
A collection of organisms of a particular species, sharing a particular characteristic of interest,
most often that of living in a given area.
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Epidemics
In epidemiology, an epidemic occurs
when new cases of a certain disease, in a
given human population, and during a
given period, substantially exceed what is
expected based on recent experience.
Epidemiologists often consider the term
outbreak to be synonymous to epidemic,
but the general public typically perceives
outbreaks to be more local and less
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serious than epidemics.
CAUSES
Epidemics of infectious disease are generally caused by a change in the ecology of the host
population (e.g. increased stress or increase in the density of a vector species), a genetic
change in the parasite population or the introduction of a new parasite to a host population
(by movement of parasites or hosts). Generally, an epidemic occurs when host immunityto a
parasite population is suddenly reduced below that found in the endemic equilibrium and
the transmissionthreshold is exceeded.
EPIDEMIC VS. PANDEMIC
An epidemic may be restricted to one location; however, if it spreads to other countries or
continents and affects a substantial number of people, it may be termed a pandemic. The
declaration of an epidemic usually requires a good understanding of a baseline rate
of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some
defined increase in incidence above this baseline. A few cases of a very rare disease may be
classified as an epidemic, while many cases of a common disease (such as the common cold)
would not.
INFLUENZA EPIDEMICS
Influenza is an infectious disease of birds and mammals caused by RNA viruses of the family
Orthomyxoviridae, the influenza viruses . The most common symptoms are chills, fever, sore
throat, muscle pains, headache (often severe), coughing, weakness/fatigue and general
discomfort. Although it is often confused with other influenza­like illnesses, especially the
common cold, influenza is a more severe disease caused by a different type of virus.
Influenza may produce nausea and vomiting, particularly in children, but these symptoms
are more common in the unrelatedgastroenteritis, which is sometimes inaccurately referred
to as "stomach flu" or "24­hour flu".
Influenza
TEM of negatively stained influenza virions, magnified approximately 100,000 times. Modern
transport contributes in spreading diseases faster.
Typically, influenza is transmitted through the air by coughs or sneezes, creating aerosols
containing the virus. Influenza can also be transmitted by direct contact with bird droppings
or nasal secretions, or through contact with contaminated surfaces. Airborne aerosols have
been thought to cause mostinfections, although which means of transmission is most
important is not absolutely clear. Influenza viruses can be inactivated by
sunlight, disinfectants and detergents. As the virus can be inactivated by soap, frequent hand
washing reduces the risk of infection.
Influenza spreads around the world in seasonal epidemics, resulting in about three to five
million yearly cases of severe illness and about 250,000 to 500,000 yearly deaths, rising to
millions in some pandemic years.
In the 20th century three influenza pandemics occurred, each caused by the appearance of a
new strain of the virus in humans, and killed tens of millions of people. Often, new influenza
strains appear when an existing flu virus spreads to humans from another animal species, or
when an existing human strain picks up new genes from a virus that usually infects birds or
pigs. An avian strain named H5N1 raised the concern of a new influenza pandemic after it
emerged in Asia in the 1990s, but it has not evolved to a form that spreads easily between
people.
In April 2009 a novel flu strain evolved that combined genes from human, pig, and bird flu.
Initially dubbed "swine flu" and also known as influenza A/H1N1, it emerged in Mexico, the
United States, and several other nations. The WorldHealth Organization officially declared
the outbreak to be a pandemic level 6 on 11 June 2009. However, the WHO's declaration of a
pandemic level 6 was an indication of spread, not severity; the strain actually having a
lower mortality ratethan common flu outbreaks.
VACCINATIONS
Vaccinations against influenza are usually made available to people in developed countries.
Farmed poultry is often vaccinated to avoid decimation of the flocks. The most common
human vaccine is the trivalent influenza vaccine (TIV) that contains purified and
inactivated antigens against three viral strains. Typically, this vaccine includes material from
two influenza A virus subtypes and one influenza B virus strain. The TIV carries no risk of
transmitting the disease, and it has very low reactivity. A vaccine formulated for one year
may be ineffective in the following year, since the influenza virus evolves rapidly, and new
strains quickly replace the older ones.
Antiviral drugs such as the neuraminidase inhibitor oseltamivir (Tamiflu) have been used to
treat influenza; however, their effectiveness is difficult to determine due to much of the data
remaining unpublished.